Leukemia relapse is the major cause of treatment failure for patients with acute lymphoblastic leukemia (ALL) (Pui et al. (2009) N Engl J Med. 360:2730-2741; Gokbuget and Hoelzer (2009) Semin. Hematol. 46:64-75; and Faderl et al. (2010) Cancer. 116:1165-1176). Relapse originates from leukemic cells that are resistant to chemotherapy but become undetectable after initial treatment in most cases. Nevertheless, methods more sensitive than microscopic examination can demonstrate leukemic cells in a proportion of samples with no morphologic evidence of leukemia, a finding termed “minimal residual disease (MRD)” (Campana (2009) Hematol. Oncol Clin North Am. 23:1083-98, vii).
MRD is currently the most powerful prognostic indicator in childhood ALL (Cave et al. (1998) N Engl J Med. 339:591-598; Coustan-Smith et al. (1998) Lancet. 351:550-554; van Dongen et al. (1998) Lancet. 352:1731-1738; Coustan-Smith et al. (2000) Blood. 96:2691-2696; Dworzak et al. (2002) Blood. 99:1952-1958; Nyvold et al. (2002) Blood. 99:1253-1258; Zhou et al. (2007) Blood. 110:1607-1611; Borowitz et al. (2008) Blood. 111:5477-5485. Basso et al. (2009) J Clin Oncol. 27:5168-5174; Canter et al. (2010) Blood. 115:3206-3214; Stow et al. (2010) Blood. 115:4657-4663). There is strong evidence supporting its prognostic significance in adult ALL (Krampera et al. (2003) Br J Haematol. 120:74-79; Vidriales, et al. (2003) Blood. 101:4695-4700; Raff et al. (2007) Blood. 109:910-915; Holowiecki et al. (2008) Br. J. Haematol. 142:227-237; Bassan et al. (2009) Blood. 113:4153-4162).
Thus, MRD monitoring has been introduced into many contemporary treatment protocols for risk assignment and selection of therapeutic regimens (Pui et al. (2009) N Engl J Med. 360:2730-2741; Gokbuget and Hoelzer. (2009) Semin. Hematol. 46:64-75; and Faderl et al. (2010) Cancer. 116:1165-1176). MRD measurements are also clinically useful in patients with relapsed ALL who achieve a second remission (Coustan-Smith et al. (2004) Leukemia 18:499-504; Paganin et al. (2008) Leukemia. 22:2193-2200; Raetz et al. (2008) J Clin. Oncol. 26:3971-3978), can help optimize the timing of hematopoietic stem cell transplantation (Bader et al. (2009) J Clin Oncol. 27:377-384), and guide decisions about donor lymphocyte infusion post-transplant (Lankester et al. (2010) Leukemia. 24:1462-1469).
Among methods for detecting MRD in ALL, PCR amplification of antigen-receptor genes has proven to be valuable and has been extensively standardized (Bruggemann, M., et al. (2010) Leukemia. 24:521-535) but the technical expertise and instrumentation required limit its application to specialized centers. PCR amplification of fusion transcripts may also provide useful clinical information but its applicability in ALL is restricted by the fact that molecular targets currently adaptable to routine MRD studies are present in only a minority of patients. Id. Flow cytometric detection of leukemia-specific markers has been shown to predict outcome in numerous clinical correlative studies (Coustan-Smith et al. (1998) Lancet. 351:550-554; Coustan-Smith et al. (2000) Blood. 96:2691-2696; Dworzak et al. (2002) Blood. 99:1952-1958; Borowitz, et al. (2008) Blood. 111:5477-5485; Basso et al. (2009) J Clin Oncol. 27:5168-5174; Krampera et al. (2003) Br J Haematol 120:74-79; Vidriales et al. (2003) Blood 101:4695-4700; Holowiecki et al. (2008) Br. J. Haematol. 142:227-237). The method holds potential for wider applicability than molecular techniques because flow cytometric methods for leukemia diagnosis are already established at most cancer centers worldwide (Campana (2009) Hematol. Oncol Clin North Am. 23:1083-98, vii).
MRD studies by flow cytometry rely on panels of antibodies to define unique immunophenotypic signatures of leukemic cells which must distinguish leukemic blasts from their normal counterparts, the CD19+ CD10+ lymphoid progenitors of the bone marrow (“hematogones”) (Campana (2009) Hematol. Oncol Clin North Am. 23:1083-98, vii; Bruggemann et al. (2010) Leukemia. 24:521-535; McKenna et al. (2001) Blood. 98:2498-2507; Lucio et al. (2001) Leukemia. 15:1185-1192).
Standard four-color flow cytometry can detect one leukemic cell in up to 10,000 normal bone marrow or peripheral blood cells but this task typically requires considerable interpretative expertise. Therefore, identification of new leukemia markers that are easily detectable and are stably expressed in a large proportion of ALL cases could simplify the application of MRD studies, and help extend their benefit to all patients and enhance the sensitivity of MRD detection.